Secondary nozzles in a combustor of a gas turbine may be permanently damaged when a flame is held in the premixing section of the nozzle. The use of high reactivity fuels makes this possibility more likely and confines operability of the gas combustor in a limited fuel space.
Use of high reactivity fuels increases flame holding risk that causes hardware damage and makes it more difficult to operate these fuels under premix operation. This has been previously addressed by so-called partially premixed design concepts that compromise mixing versus flame holding risk and increases NOx emissions.
Referring to FIG. 1, an exemplary gas turbine 12 includes a compressor 14, a dual stage, dual mode combustor 16 and a turbine 18 represented by a single blade. Although not specifically shown, the turbine 18 is drivingly connected to the compressor 14 along a common axis. The compressor 14 pressurizes inlet air which is then turned in direction or reverse flowed to the combustor 16 where it is used to cool the combustor and also used to provide air to the combustion process. The gas turbine 12 includes a plurality of the combustors 16 (one shown) which are located about the periphery of the gas turbine 12. A transition duct 20 connects the outlet end of its particular combustor 16 with the inlet end of the turbine 18 to deliver the hot products of the combustion process to the turbine 18.
Referring to FIGS. 1 and 2, each combustor comprises a primary or upstream combustion chamber 24 and a second or downstream combustion chamber 26 separated by a venturi throat region 28. The combustor is surrounded by a combustor flow sleeve 30 which channels compressor discharge air flow to the combustor. The combustor is further surrounded by an outer casing 31 which is bolted to the turbine casing 32.
Primary nozzles 36 provide fuel delivery to the upstream combustion chamber 24 and are arranged in an annular array around a central secondary diffusion nozzle 38. Each combustor may include six primary nozzles and one secondary nozzle, although it should be appreciated that other arrangements may be provided. Fuel is delivered to the nozzles through plumbing 42. Ignition in the primary combustor is caused by spark plug 48 and in adjacent combustors by crossfire tubes 50.
Referring to FIG. 2, a primary diffusion nozzle 36 includes a fuel delivery nozzle 54 and an annular swirler 56. The nozzle 54 delivers only fuel which is then subsequently mixed with swirler air for combustion. The centrally located secondary nozzle 38 contains a major fuel/air premixing passage and a pilot diffusion nozzle.
During base-load operation, the dual stage, dual mode combustor is designed to operate in a premix mode such that all of the primary nozzles 36 are simply mixing fuel and air to be ignited by the secondary premixed flame supported by the secondary nozzle 38. This premixing of the primary nozzle fuel and ignition by the secondary pilot diffusion nozzle leads to a lower NOx output in the combustor.
Referring still to FIG. 2, a diffusion piloted premix nozzle 100 includes a diffusion pilot having a fuel delivery pipe. The diffusion pilot further includes an air delivery pipe coaxial with and surrounding the fuel delivery axial pipe portion. The air input into the air delivery pipe is compressor discharge air which is reverse flowed around the combustor 16 into the volume 76 defined by the flow sleeve 30 and the combustion chamber liner 78. The diffusion pilot includes at its discharge end a first or diffusion pilot swirler for the purpose of directing air delivery pipe discharge air to the diffusion pilot flame.
A premix chamber 84 is defined by a sleeve-like truncated cone which surrounds the diffusion pilot and includes a discharge end (as shown by the flow arrows) terminating adjacent the diffusion pilot discharge end. Compressor discharge air is flowed into the premix chamber 84 from volume 76 in a manner similar to the manner in which air is supplied to the air delivery pipe. The plurality of radial fuel distribution tubes extend through the air delivery pipe and into the premix chamber 84 such that the injected fuel and air are mixed and delivered to a second or premix chamber swirler annulus between the diffusion pilot and the premix chamber truncated cone. Further details of the combustor and gas turbine engine shown in FIGS. 1 and 2 are disclosed in, for example, U.S. Pat. No. 5,193,346